Mineralogy and Petrology最新文献

This paper reports the metamorphic texture of cordierite megacrysts and the metamorphic P–T path of a newly exposed section of gneiss in East Antarctica. We used mineral textures and pseudosection modeling to reconstruct the metamorphic P–T path of cordierite- and spinel–garnet-bearing gneisses from Botnnuten, an isolated nunatak located ~ 60 km from the southern edge of Lützow-Holm Bay in East Antarctica. The gneisses underwent low-P granulite-facies metamorphism at 5.0–6.1 kbar and 850 ± 20 °C followed by isobaric cooling. The isobaric cooling path implies long residence in the middle to shallow crustal level without rapid exhumation. This contrasts with the widely recognized clockwise P–T path of basement rocks of the Lützow-Holm Complex. The rocks at Botnnuten have long been considered part of the Lützow-Holm Complex based on their petrographical features and geothermobarometric data. However, the present results, combined with a reevaluation of available data, indicate the metamorphic history of the Botnnuten gneisses is more comparable to that of the Yamato Mountains, located southwest of the study area.

This study presents and discusses first detailed petrographic, microthermometric and Raman spectroscopic data from quartz-hosted fluid inclusions at Rudnik Zn-Pb-Cu-Ag skarn deposit (Serbia) and combines them with the information on skarn- and ore paragenesis. Three periods in the metamorphic-hydrothermal history of the deposit are recognized: 1) the pre-ore prograde skarn period when garnet-clinopyroxene skarns formed, 2) the syn-ore period that encompasses a retrograde stage marked by epidote and zoisite and a quartz-sulfide stage characterized by quartz, pyrrhotite, sphalerite, galena and chalcopyrite, and 3) the post-ore period associated with precipitation of calcite and quartz. The hydrothermal evolution is inferred from studying six groups of quartz-hosted fluid inclusions (FI). Two-phase FI of high- (Group A) and moderate salinity (Group B) are found in quartz cores and homogenized at 380–390 °C (mode) and 370–380 °C (mode), respectively. Group A FI consists of H₂O-NaCl liquids and CO₂-CH₄ gas mixtures and likely represents the original fluid composition, whereas Group B FI records dilution of the original fluid at constant temperature, with a slight increase in CH₄ contents. The quartz cores also contain Group C as volatile-rich FI (mostly CO₂ with up to 10 mol% CH₄ and H₂S) of a moderately low salinity and liquid-rich Group D FI composed of pure water with homogenization temperatures of 180–200 °C (mode). The transitional zones of quartz crystals show overgrowth textures and host Group E FI with low salinity that homogenized at 235–401 °C, which vapour phase is a CO₂-CH₄ mixture with up to 17 mol% CH₄. Group F comprises FI found within the rim zones of quartz crystals and they exhibit a low salinity and homogenization temperatures between 259–365 °C. Accordingly, the hydrothermal history at Rudnik involved: a) mixing of different salinity fluids at high temperatures (Groups A and B—retrograde stage), b) introduction of fluids with high volatile contents (Group C) and cooling of fluids with constant salinity (between Groups E and F), which likely correspond to the quartz-sulfide stage, and c) inflow of meteoric water (Group D—the post-ore quartz-calcite stage).

Natural corundum shows two types of twins: “basal twin”, by reflection on (0003) pinacoid, very rare, and “rhombohedral twin”, by reflection on ((10overline{1 }1)), more frequent. The analysis of the structural continuity across the composition plane does not show any reason for a large difference in occurrence frequency, which is likely related to the limited development of the (0003) plane in the characteristic morphology of corundum. “Basal twins” occur with unusually high frequency in samples from Greenland, which also present an atypical platy morphology, where the (0003) face is well developed. This observation seems to confirm a morphological control on the occurrence of the “basal twin”. All analysed twinned samples show macrosteps on their pinacoidal faces and this feature has been related to the high-temperature conditions and intense fluid-rock interactions of Greenland deposit. This clearly suggests a strong relationship between the “basal twin” occurrence, the development of basal faces, and the formation conditions. However, due to the complex geological context and the different features of samples (e.g. two individuals with almost the same size versus several lamellae stacked along c axis), it is not possible to establish with certitude if the “basal twins” observed in Greenland samples are growth or mechanical twins.

Rapakivi granite is characterized by its unique structure, which has important implications for tectonic settings, magmatic processes, and crust–mantle interactions. In this study, we conducted a combined analysis of the petrography, mineral chemistry, geochemistry, and zircon U–Pb dating and Lu–Hf isotopic compositions of the Niujiaoshan Early Paleozoic rapakivi-textured granite from the North Qinling Belt. Zircon U–Pb dating yielded a crystallization age of 447 ± 7 Ma, which is younger than the ultra-high-pressure (UHP) metamorphic age (~ 500 Ma) but similar to the granulite facies retrograde age (~ 450 Ma) of UHP eclogites and felsic gneisses in the North Qinling Belt. The rapakivi feldspar phenocrysts have ovoid K-feldspar cores, which are rich in mineral inclusions, such as amphibole, biotite, quartz, and plagioclase, indicating early crystallization. The ovoid K-feldspar cores are mantled by oligoclase, whreras the matrix comprises biotite, amphibole, and coarse-grained plagioclase. The amphibole and biotite in the granite are rich in Mg and are indicative of a crust–mantle origin. The ε_Hf (t) values of the zircons range from − 2.04 to + 3.63, suggesting formation via crust–mantle interactions. The rapakivi-textured granite displays high-K meta-aluminous I-type granite affinity, with high SiO₂, K₂O, and Na₂O contents. Based on the geological background and results of this study, we propose that the Niujiaoshan rapakivi-textured granite was formed via the mixing of crustal materials induced by upper mantle magma during the exhumation of the North Qinling UHP metamorphic terrane, which occurred in a post-orogenic setting.

Occurrence of “ferrian chromites” have earlier been reported from the Mesoarchean chromite deposits in the Boula-Nuasahi ultramafic complex (BNUC) of India. We have investigated the chromitite bodies in the southern part of the BNUC (i.e., Bangur area) with respect to the mode of occurrence, petrography, chemistry and structure of the chromite types. Although morphologically five varieties of chromite ore were found, chemically only three types of chromite can be distinguished based on EPMA analysis. These are: 1) Type I: magnesiochromite [high Cr₂O₃ (57–65 wt.%), low iron (FeO: 13–17 wt.%), X^Fe3+: < 0.1 apfu]; 2) Type II: ferrian chromite [moderate Cr₂O₃ (43–53 wt.%), high iron (FeO: 27–30 wt.%), X^Fe3+: 0.1 to 0.5 apfu]; and 3) Type III: ferrichromite [low Cr₂O₃ (19–29 wt.%), very high iron (FeO: 55–67 wt.%), X^Fe3+: 0.5 to 1.0 apfu]. Stoichiometrically calculated Fe₂O₃ content is very high in some grains (maximum 47 wt.%). Geochemical discrimination diagrams for the Type I pristine magnesiochromite suggest a dominantly boninitic parental magma. Trace element data obtained from LA-ICP-MS indicate that the Type II chromite has formed from a more evolved magma and is richer in trace elements such as V, Mn, Co, Cu, Pb, Ga, and Nb whereas the Type III ferrichromite shows unusually high Ti and erratic high concentrations of trace elements. Alterations in chromite is noticed in two different thermal regimes: 1) 100–200 °C related to serpentinization of dunite and peridotite rocks where chromite grains show an unaltered core, an intermediate ferrian chromite rim and an outer magnetite rim; 2) 500–600 °C where the entire chromite grain is converted to ferrichromite which can be linked to later intrusion of the Bangur gabbro. While HR-TEM study reveals that all three chromite-types have face-centered cubic structure, Raman spectroscopy indicates that there is a gradual transition of the structural state from normal spinel structure (Type I) through Type II to a fully inverse spinel structure in case of ferrichromite.

For a long time, it has been implicitly believed that oxygen isotopes of hydrothermally altered rocks and/or minerals were only elevated by the heavy water enriched in ¹⁸O from the modern geothermal and/or fossil hydrothermal systems around the world. While it is logically likely, there is no any previous attempt to argue for the elevation of oxygen isotopes of hydrothermally altered rocks and/or minerals by a light water depleted in ¹⁸O under appropriate natural conditions. Based on a novel procedure recently proposed for dealing with thermodynamic reequilibration of oxygen isotopes between constituent minerals and water, the initial oxygen isotopes of water (i.e., (delta {^{18}{text{O}}}_{text{W}}^{text{i}}) value hereafter) prior to the hydrothermal alteration are theoretically inverted from the early Cretaceous postcollisional granitoid and Triassic gneissic country rock across the Dabie orogen in central-eastern China. The oxygen isotopes of hydrothermally altered rock-forming minerals were concurrently elevated by the magmatic water with moderate to high ({delta ^{18}{text{O}}}_{text{W}}^{text{i}}) values ranging from 4.21 ± 0.04 (one standard deviation, 1SD) to 6.57 ± 0.05‰ in the course of postmagmatic processes. By contrast, oxygen isotopes of the susceptible alkali feldspar from a gneissic country rock could be preferentially elevated by the ancient meteoric water with low (delta {^{18}{text{O}}}_{text{W}}^{text{i}}) values down to -8.52 ± 0.56‰ during exhumation processes of the retrograde metamorphism. These fossil hydrothermal systems could kinetically sustain from a short duration of less than 12 thousand years (Kyr) via the surface-reaction oxygen exchange up to 1 million years (Myr) through the diffusive oxygen exchange, respectively, in this study. Cooling rates are further quantified for rock-forming minerals sequentially blocked and/or isolated from the magmatic water. Hereby, oxygen isotopes of constituent minerals can be hydrothermally elevated by diverse sources of water with paradoxical (delta {^{18}{text{O}}}_{text{W}}^{text{i}}) values, especially for the metamorphic rocks with anomalous oxygen isotopes. There is no doubt that more unexpected findings will be scientifically and methodologically decoded and/or unlocked worldwide in the coming decade(s).

This paper addresses the study of a pillow lava interbedded with Late Albian-Early Cenomanian sediments that crops out in Armintza (Bizkaia, Northern Spain). The lava flow is an alkaline basalt with abundant macrocrysts of clinopyroxene, kaersutite, Ca-rich plagioclase (50-86% An) and ilmenite, which display a variety of textures and complex zoning patterns indicative of open-system magmatic behaviour. Macrocryst cores are likely to be inherited antecrysts that underwent complex processes under deep pre-eruptive conditions (≈ 700-800 MPa). Microcrysts and macrocryst rims formed during magmatic ascent and emplacement at shallower levels (≈ 35 MPa). Hypothetical melts in equilibrium with clinopyroxenes and amphiboles have trace element compositions like metasomatic vein melts containing amphibole, and their patterns overlap with those of the Armintza pillow lava. This suggests a metasomatised lithospheric mantle with amphibole-rich veins as a potential source for the alkaline basaltic melt. It is even conceivable that the Armintza pillow lava and other alkaline volcanic manifestations of the Basque-Cantabrian Basin were part of the same magma plumbing system through which a series of time-limited eruptions of different batches of magma ascended from the lithospheric mantle to the upper crust during the Albian to the Santonian.

As a tectonic window into the Lesser Himachal Himalaya, India, a group of metasediments and gneissic rocks, known as the Jutogh Group and Wangtu Gneissic Complex (WGC), occurs near the Jhakri thrust to the west and Wangtu to the east. In the Jutogh Group, chlorite-mica schist, garnet-staurolite schist and sillimanite-schist develop successively. The formation of chemically zoned garnet, which destabilized low-temperature assemblages, is predicted to be at 550–650 °C and 0.8–0.9 GPa by phase equilibria modelling. The retrograde segment consists of exhumation and cooling, yielding a tight clockwise P–T path. Moreover, textural observations and in-situ U-Th-Pb chemical dating indicate that metasedimentary rocks contain Cambrian monazites. These monazites have ages that cluster around 500 Ma. The Ɛ_Nd[1.8Ga] of Jutogh rocks ranges from − 1.0 to -8.1, with depleted mantle-model ages between 3.07 and 2.25 Ga. The garnet core and its leachates yield an Sm-Nd isochron age of 472 Ma. Another Sm-Nd isochron age of 454 Ma is obtained from biotite, garnet rim, and garnet rim leachate. According to phase equilibrium modelling, Sm-Nd dating, and monazite geochronology, the Jutogh Group experienced metamorphism along the northeast margin of Gondwana during the Cambro-Ordovician accretion.

Metamorphosed banded iron formation (BIF) in granulite-amphibolite facies, tonalitic orthogneisses from a series of locations in the Kolli Massif of southern India are described and analysed with regard to their lithologies, whole rock chemistry, mineral reaction textures, and mineral chemistry. On the basis of their mineral reaction textures along magnetite-quartz grain boundaries these BIFs are grouped according to their predominant silicate mineralogy: 1) amphibole; 2) orthopyroxene; 3) orthopyroxene–clinopyroxene; 4) orthopyroxene-clinopyroxene-garnet; 5) clinopyroxene-garnet-plagioclase; and 6) Fe-Mg silicates are absent. Two-pyroxene and garnet-pyroxene Fe-Mg exchange thermometry, coupled with thermodynamic pseudo-section modelling of whole rock data from one of the magnetite-quartz-orthopyroxene-clinopyroxene-bearing lithologies, indicates that the magnetite-quartz-orthopyroxene-clinopyroxene-garnet assemblages formed at ~900 to 1200 MPa and 750 to 900 °C under relatively low H₂O activities. Magnetite-quartz-orthopyroxene reaction textures were experimentally replicated at 800 and 900 °C and 1000 MPa in a synthetic BIF using isolated magnetite grains in a quartz matrix to which was added a hypersaline Mg- and Al-bearing fluid (approximately 1% by mass), which permeated along all the grain boundaries. The fact that Fe-Mg silicate reaction textures did not form in one of the BIF samples, which had experienced the same P-T conditions as the other BIF samples, suggests that, unless a BIF initially incorporated Mg, Al, and Ca during formation with or was infiltrated from the surrounding rocks by Mg-, Al-, and Ca-bearing saline fluids, these silicate minerals could not and would not have formed from the inherent magnetite and quartz during granulite-facies and amphibolite-facies metamorphism.

The Tirodi Gneissic Complex (TGC) represents the basement sequence of the Central Indian Tectonic Zone (CITZ), underlying the Proterozoic supracrustal sequences of the Sausar and Betul Groups of rocks. Lithologically, the TGC constitutes a combination of pink and grey granitic gneiss assemblages, characterised by biotite-rich, hornblende-biotite-rich, and muscovite-biotite-rich granite gneiss. Compositionally, the TGC granitoids represent tonalite-trondhjemite-granodiorite to granite, and have calc-alkaline lineage with metaluminous to peraluminous characteristics. Geochemically, they dominantly belong to A2-type granitoids. Chondrite normalised REE ratios of La/Sm, La/Yb, La/Gd, and Gd/Yb indicate diverse LREE/HREE enrichment. Multi-element patterns for the TGC granitoids are characterised by light rare earth elements (LREE) and large ion lithophile elements (LILE) enrichment and depletion of high field strength elements (HFSE: Nb, P, and Ti) and strong positive Pb and Th anomalies. The observed negative anomalies for HFSE are attributed to diverse crustal/lithospheric sources, with some influence from K-feldspar, plagioclase and Ti-oxide fractionation. Sm–Nd data presents initial ¹⁴³Nd/¹⁴⁴Nd (t = 1.7 Ga) ratios (0.509898 to 0.510508), and ε_Nd (t = 1.7 Ga) is (+ 0.58 to -10.59), with T_DM model ages ranging from 2.11 to 2.95 Ga. Such a wide range of ε_Nd (t = 1.7 Ga), indicates heterogeneous crustal/lithosphere sources, which have probably experienced longer crustal residence times. Zircon U–Pb ages for individual TGC samples are 1506 ± 11 Ma (TG-01), 1534 ± 26 Ma (MU-5), 1675 ± 9 Ma (BT-4), 1724 ± 11 Ma (BT-3), 1730 ± 13 Ma (BT-4), and 1960 ± 2 Ga (Ms-2), respectively. These ages have probably recorded the key periods of the Columbia supercontinent's assembly, growth, and breakup. Geochemical and geochronological results suggest that the TGC granitoids have a crustal/lithospheric origin and are formed by partial melting of felsic sources in dominantly VAG (volcanic arc granite) and, to some extents, WPG (within-plate granite) settings.